I can't say I have ever noticed this, but I do distinctly remember a conversation with two close friends of mine in the last month who are both pot heads, they were both of the opinion that they go to the toilet more regularly when they are on the choof than when they are taking a break from smoking. It is worth noting that both of them mix tobacco with their cannabis so this effect could be attributed to the nicotine, although one in particular does smoke cigs and he smokes more of them if he isn't on the weed so it is hard to say.
I agree that stimulants definitely increase ones need to shit, whether you are talking nicotine, caffeine, cocaine, meth, anything. All of 'em make me need to take a shit, even anticipating meth use these days makes me need a shit because I get all anxious.